One of the challenges of wireless communications, is designing suitable antennas that provide desired performance characteristics, yet are relatively small in size to fit within mobile devices. For example, with wireless devices such as mobile telephones, it is desirable to maintain the overall size of the telephone as small as possible. Furthermore, internal antennas are generally preferred over external antennas, as externally mounted antennas take up more space and may be damaged while traveling or through other uses.
These wireless devices often operate with cellular communication systems that continue to grow in popularity and have become an integral part of both personal and business communications. Moreover, as cellular telephone technology increases, so too has the functionality of the devices. For example, many portable wireless communications devices now incorporate Personal Digital Assistant (PDA) features such as calendars, address books, task lists, calculators, memo and writing programs. These multi-function devices usually allow users to send and receive electronic mail (email) messages wirelessly and access the internet via a cellular network and/or a wireless local area network (WLAN), for example, when the devices include appropriate circuitry for WiFi and other IEEE 802.11 WLAN access. Many of the cellular communications use packet burst transmissions as part of a Global System for Mobile communications (GSM) system, which includes the 850 MHz, 900 MHz, 1800 MHz and 1900 MHz frequency bands. Although these mobile wireless communication devices function as a cellular telephone, as noted before, the device can also operate and incorporate Personal Digital Assistant (PDA) features and send and receive email and other messages wirelessly and across the internet via the cellular network and/or a wireless Local Area Network (LAN). This function can include access to “hot spots” as part of a WiFi network using IEEE 802.11 standards.
Recent carrier specifications stipulate the addition of diversity antenna in the new generation of wireless mobile communications devices. In order to achieve acceptable diversity performance however, the radiating elements must be electromagnetically isolated. In a mobile wireless communications device having a handheld form factor, achieving adequate isolation often is difficult depending on the specific designs. In some devices, there are two antennae in close proximity to each other that operate in the same frequency spectra (850 and 1900), for example. This configuration results in strong coupling between the two antennae and degrades the radiated performance as they interfere with each other destructively. A possible solution is to tune the destructive interference into another mutually exclusive operating frequency band, which does not require diversity (such as 900 and 1800) or if possible, outside of any operating frequencies. In a multi-band portable wireless communications device, moving the interference into a non-diversity band is insufficient since the interference remains and degrades performance in the non-diversity band.
As an example, in one mobile wireless communications device, diversity is required in the 850 and 1900 bands, but not in the 900 and 1800 bands. Furthermore, the 850 and 1900 bands are mutually exclusive of the 900 and 1800 bands because they do not operate simultaneously. If the interference is tuned to the 900 band, for example, acceptable diversity performance can be achieved. However, the problem may not be solved because if the handheld is operating in the 900 band, the interference could remain. In order to maintain the antenna performance in the 900 band, the diversity antenna should be electromagnetically invisible to the main antenna while operating in the non-diversity bands.